This invention generally relates to tape guides for use in magnetic tape recording and playback machines and particularly concerns a tape guide constituting part of a tape loop sensing arm in a tape transport forming part of a machine for recording or playing back video signals.
In most kinds of magnetic tape recording and playback machines, magnetic tape is transported along a path defined by a plurality of guides from a supply reel of tape past a recording head to a take-up reel. The tape is usually driven by a capstan engaging the tape at an appropriate location along the path, the location depending on the particular machine. The supply and take-up reels are driven, usually by means of motors which are controlled servomechanically to maintain an appropriate tension in the tape. In video machines, the tape path extends adjacent a rotary scanner which, for helically scanning machines, comprises a drum around which the tape is guided in a helical path so as to be scanned in oblique tracks by a scanning head driven at high speed around the periphery of the drum so as to provide scanning of the tape along a plurality of oblique tracks on the tape. In all kinds of magnetic tape recording and playback machines it is desirable that the speed of the tape be accurately controlled but particularly in video machines it is, as is well-known, highly desirable to control the motors used for driving the supply and take-up reels to maintain a substantially constant tape tension and to avoid loading the capstan by the reels.
In order to monitor the tension of the tape along a section thereof adjacent one or other of the supply and take-up reels, it is known to provide an arm which is provided at or near one end and carries at or near its other end a roller guide around which the tape extends. By means of guides upstream and downstream of the arm, the tape path extending around the guide carried on the arm forms a small loop. The pivoting of the arm enables the guide carried by the arm to move along, for example, an arcuate path and the arm is usually biassed so as to tend to make the loop of the tape of maximum length. It is usual to provide two such pivoted arms and associated guides, one each being disposed near the respective supply or take-up reel. Considering the arm associated with the supply reel, for the sake of example, as the tension in the loop of tape formed by the guide carried on the pivoted arm increases, the pressure of the tape on the guide will increase so as to move the guide along said arcuate path, which usually extends in a plane to which the axis of rotation of the guide and the axis about which the arm carrying the guide can pivot are normal. The consequential pivoting of the arm can be sensed, by a variety of means not directly relevant to the present invention, to obtain a signal representing the loop length and thereby the tape tension. As is also known, such a signal can readily be used as part of a servo-loop controlling the motor for the supply reel to accommodate variation in the tape tension. Thus the supply reel's motor can be driven so as to accelerate or decelerate as necessary and thereby to relax or increase the tension applied to the tape by the supply reel and thereby to maintain the tape tension, as monitored by the pivoted arm, substantially constant. It will be readily apparent that converse considerations apply for the tension sensing arm associated with the take-up reel and that the circuits for driving the supply and take-up reels include mode switching arrangements for controlling the motor differently according to the particular mode of operation of the transport that may be required, such as for example high speed traversal or rewind etc.
Owing to the considerable pressure exerted by the tape on the guide carried by the tension sensing arm, the bearings for the guide are liable to wear and it is found in practice that if the rotary guide is mounted by simple bearings on a post, the bearings develop undesirable play such that the tape guiding is inaccurate. Moreover, the wear of the tape can be considerable and it would be generally desirable to be able to accommodate slight variations in the height or tension in the tape so as to reduce wear without substantially affecting the accuracy of the sensing arm.